1. Field of the Invention
The present invention relates to a method for controlling the mains bridge of a four-quadrant frequency converter and to a mains bridge.
2. Description of Background Art
Adjustable electric motor drives controlled by a frequency converter often need to have a braking ability. The braking energy can be fed either into a resistor, which will convert it to heat, or back to the supply network, thus allowing it to be utilized elsewhere. Regeneration of braking energy to the mains is considerably more expensive to implement than resistor braking, so it is only advantageous to use when the braking energy saved is sufficiently significant in an economical sense.
An advantageous possible way of implementing regenerative braking is to use between the supply mains and the DC intermediate circuit a full-wave bridge circuit (a so-called FFE unit) controlled by the mains frequency. The FFE unit consists of an inverter unit and an AC inductor. FIG. 1 presents a four-quadrant FFE unit of a three-phase voltage-controlled PWM frequency converter, said unit comprising a controlled mains bridge INU connected to the supply network L1–L3 for rectifying the mains voltage to produce a DC intermediate-circuit direct voltage UDC. The mains bridge consists of arms conducting during the positive and negative half cycles of the phase voltage of each supply network phase, each arm having a switch unit formed by a diode D1–D6 and a power semiconductor switch V1–V6, e.g. an IGBT, connected in inverse-parallel with it. The diodes are conducting when power is flowing from the mains into the motor, and the semiconductor switches are conducting during regenerative braking when power is flowing from the motor into the mains. The power stage of the inverter is identical to that in the motor feeding application, differing from it in the FFE application only in respect of the control system. An AC inductor LAC is used as a filter to limit the mains current harmonics.
In regenerative braking (generator use), the power semiconductor switches of the FFE are controlled in such a way that the switch V1, V2, V3 (conducting during the positive half cycle) in the upper arm of each phase is turned on substantially for as long a time as the supply network phase voltage UL1, UL2, UL3 corresponding to it is the most positive of all the phase voltages, and similarly the switch V4, V5, V6 (conducting during the negative half cycle) in the lower arm is turned on substantially for as long a time as the corresponding phase voltage is the most negative of all the phase voltages. FIG. 2 illustrates this type of control of the power semiconductor switches. In a motor drive, the power semiconductor switches of the FFE need not necessarily be controlled at all because the current flows continuously through the diodes.
The method of controlling the FFE requires synchronization with the supply network. A prior-art method of implementing the synchronization is to measure the network voltages by means of a specific auxiliary card and feed the measurement data to the control unit of the FFE. The voltage measurement is thus of a nature susceptible to disturbances, which, besides the operation of the FFE's own power switches, may also be caused by other devices connected to the same supply network. Especially in weak networks, the voltage to be measured and therefore also the measurement signal may contain many disturbances, impairing synchronization accuracy. The effect of disturbances can be dampened by filtering the measurement result, but this again induces a phase shift in the measurement signal, which also makes accurate synchronization with the network more difficult.